The sealing of substantially open-cell polyurethane elastomers prevents water, dust and dirt from penetrating into the cellular polyurethane elastomer, surprisingly without affecting the physical properties, resilience for example, of the elastomer.
Moldings of cellular polyurethane elastomers are commercially produced in known manner from polyisocyanates, relatively high molecular weight polyhydroxyl compounds, water as chain extender and blowing agent and, optionally, additional chain extending agents. Cellular polyurethane elastomers are distinguished from the polyurethane foams by considerably higher densities (approximately 0.3 to 0.8 g/cc) and improved physical properties. These improvements create new commercial applications for polyurethanes. For example, high quality cellular polyurethane elastomers, such as produced from 1,5-naphthylene diisocyanate, a linear ethane diol polyadipate (molecular weight approximately 2,000) and water, are used commercially inter alia as shock absorbing and damping materials.
One significant field of application is in the motor vehicle industry where materials of the type in question are used primarily for damping and shock absorbing.
The replacement of the rubber auxiliary springs in spring-leg assemblies with auxiliary springs made of cellular polyurethane elastomers is advantageous because of the greater deformability of cellular polyurethanes. Deformations of cellular polyurethanes of up to 80% are quite possible in practice (for example, in auxiliary springs in automobiles).
In automobiles, the cushioning elements made of cellular polyurethane elastomers are fitted onto the piston rod of the shock absorber within the overall spring-leg assembly consisting of shock absorber, helical spring and the damping element of cellular elastomer. With this arrangement, the damping elements are frequently in danger of becoming soiled because splashed water and dirt are able to penetrate into the damping elements because of their predominantly open-cell pore structure. This can give rise to changes in the cushioning effect, especially at low temperatures. Additionally, the piston rod of the shock absorber also undergoes premature corrosion and wear under the influence of water and dirt held in the damping elements.
The object of the present invention is to provide cushioning elements which may be produced from known formulations and by means of the processing machines normally used in practice which not only show the known favorable cushioning properties, but are also water-tight.
It has now been found that this object may be achieved by providing the known damping elements of elastomeric cellular polyurethane with a tight, impervious skin of polyurethane or polyurethane urea.
Cellular polyurethane elastomers differ from integral foams in that their density is substantially uniform over their entire cross section. Although a tighter outer layer may be obtained by lowering the molding temperature, this adversely affects the cushioning behavior and uniformity of the articles. In addition, the lowering of the molding temperature does not afford complete protection against penetrating moisture and particles of dirt because the outer layer thus produced is not completely pore-free.
It had been assumed that a separately produced, tight surface skin on the damping element would also affect its cushioning behavior because an impervious skin applied to the foam would deprive it of its ability to breathe which results from the open-pore cell structure.
However, it has surprisingly been found that the cushioning behavior of the damping element is not affected by such an outer skin.